1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.
In this specification and the like, a “semiconductor device” generally refers to a device which can function by utilizing semiconductor characteristics: an electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
A technique in which a transistor is formed using a semiconductor layer has attracted attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). As semiconductor materials applicable to the transistor, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.
For example, it is confirmed that an oxide semiconductor including an In—Ga—Zn oxide is applicable to a channel formation region of a thin film transistor (Non-Patent Document 1). Non-Patent Document 1 discloses a top-gate transistor including a hafnium oxide layer which is over and in contact with an oxide semiconductor layer including an In—Ga—Zn oxide and which is used as a gate insulating layer and an indium tin oxide layer which is over and in contact with the hafnium oxide layer and which is used as a gate electrode layer.
As a method for controlling the threshold voltage of a transistor using a silicon semiconductor, a method in which impurity elements imparting one conductivity are added to a region in which a channel is formed (channel doping) is known.
Meanwhile, it is known that in the case where oxygen vacancies exist in an oxide semiconductor of a transistor using an oxide semiconductor, part of the oxygen vacancies becomes donors and causes release of electrons serving as carriers. When the electrons serving as carriers are released, a channel is formed in the transistor without application of voltage to a gate, whereby the threshold voltage is shifted in the negative direction. Since it is difficult to completely eliminate oxygen vacancies included in the oxide semiconductor, the threshold voltage of the transistor using the oxide semiconductor is hardly controlled; thus, the transistor tends to be normally on.
A method for suppressing shift in threshold voltage of a transistor using an oxide semiconductor in negative direction has been studied; however, it is hard to say that a method for causing shift in the threshold voltage in the positive direction has been established.
It is known that in an MIS structure including a silicon semiconductor, negative charges are trapped in trap centers in an insulating layer formed over a semiconductor layer, and thus flat band voltage is shifted (changed) in the positive direction, and a cluster of metal is reported as a trap center (see Non-Patent Document 2).